In addition to hydroprocessing, other techniques have been disclosed for the removal of nitrogen compounds. Recently, two U.S. Pat. Nos. issued to Baset, 4,332,676 and 4,332,675, which disclose a process for the removal of basic nitrogen compounds from organic streams inclusive of petroleum oils utilizing gaseous sulfur dioxide to thereby precipitate a salt comprising the basic nitrogen compound, sulfur dioxide and water with downstream separation of the precipitated salt. Both of these patents concern a single phase treatment system with an essentially water-free separation system in '675 and only enough water in '676 such that a single phase system is existent. In fact, in the latter reference, the addition of water is limited to a concentration only to the extent that a two-phase liquid system will never be formed. It is also disclosed that a non-polar solvent can be utilized in the contacting step such as petroleum ether, a lower paraffinic hydrocarbon or an aromatic hydrocarbon such as toluene. While the types of basic organic nitrogen compounds extracted in the first step of the instant invention are either similar to or the same as those described in Column 2 of the '676 disclosure, the means by which the process is undertaken in the instant invention is very different from that disclosure.
In the October 1983 issue of Chemical Engineering, an article by Desai and Madgavkar recognized a method to remove nitrogen compounds from shale oil by solvent extraction with a formic acid/water solvent prior to hydrotreating. The advantage of this technique is a reduced hydrogen consumption and a reduction of the nitrogen content to a level that allows downstream processing of the shale oil.
The use of inorganic acids to petroleum oils to reduce the quantity of nitrogen compounds has long been established. For example, in U.S. Pat. No. 2,352,236, anhydrous hydrogen chloride is added to improve a charge stock for catalytic cracking. A dilute acid, such as sulfuric acid, is disclosed in U.S. Pat. No. 1,686,136 to complex nitrogen compounds existent in a California-derived crude oil. Organic carboxylic acids, sometimes referred to as low molecular weight fatty acids of high volatility, have been used to complex nitrogen-bases in such disclosures as U.S. Pat. Nos. 2,263,175 and 2,263,176. While these latter two references employ a portion of the chemical mechanism utilized in the first step of this two-step nitrogen extraction process they fail to disclose, suggest or even hint at the use of a second extraction step with an immiscible hydrocarbon solvent to improve the selectivity of the process, which results in a higher overall yield of denitrogenated raffinate. Also, these references fail to teach the use of two extraction steps, one utilizing an acidic solvent and one utilizing an immiscible hydrocarbon solvent.
A patent issued to Madgavkar et al, U.S. Pat. No. 4,671,865, discloses a two step process for removing heterocyclic nitrogen from petroleum oils. The patent describes the combination of hydrotreatment followed by acid extraction for removing nitrogen from petroleum oil.
U.S. Pat. No. 4,426,280 discloses the removal of nitrogen from shale oil by contacting an oil stream with a dilute acid followed by contacting the oil stream with a concentrated acid.
A patent issued to Johnson et al, U.S. Pat. No. 4,409,092, in 1983 teaches formation of a high nitrogen fraction and a low nitrogen fraction, which is then subjected to phosphoric acid extraction. The fraction high in nitrogen content is catalytically cracked and then either hydrotreated or sent to phosphoric acid extraction. There is no disclosure by Johnson et al of a two-step extraction process whereby extraction of petroleum oil is made in the presence of a concentrated acid extraction agent and then subsequently the extract from the first extraction is contacted with an immiscible hydrocarbon solvent, thus improving the overall yield of denitrogenated raffinate.
Thus, the extraction of basic heterocyclic nitrogen compounds from petroleum oils using acidic solvents is an established technique. However, such solvents extract not only nitrogen containing materials from the oil, but can dissolve substantial amounts of nitrogen-free hydrocarbon as well. Nitrogen-free hydrocarbon adsorption by the acid phase constitutes a loss in raffinate yield. Such losses are particularly high when a high percentage of the basic nitrogen present in the feed must be removed. Accordingly, it has been found that a specific two step extraction procedure can substantially improve process selectivity and denitrogenated raffinate yield.